1. Field of the Invention
The present invention relates to an active steering mechanism for steering an endless belt supported by a plurality of rotatable rollers, and in particular to a steering roll construction which is capable of actively correcting lateral belt misalignment, correcting for "belt walk" (lateral displacement) and "walk rate" (lateral velocity) while all portions of the belt surface, including unsupported sections between guide rollers, are undisturbed when correction is applied.
2. Description of Related Art
Printers and copiers use various active and passive guiding methods for wide belts and webs. Passive designs rely on a balance of forces to restrict the belt to a well defined path. Active systems sense belt position and mechanically realign the guide rolls to steer the belt to a fiducial position.
Passive systems may include hard edge guides which force an edge of the belt to remain on a defined path. This has the disadvantages of not precisely controlling the exact location of the belt and in extreme cases can cause damage to the edges of the belts.
Another passive system is shown in U.S. Pat. No. 2,593,158 which discloses an apparatus for positioning strips. The apparatus includes two spaced-apart rotatable frusto-conical rolls located on a shaft over which a strip or conveyor belt is passed. The frusto-conical rolls are connected to the shaft through loosely fitted keys or spherical bearings. Due to the way the rolls tilt on the shaft independently of one another, corrective forces are exerted on the belt when the belt is not symmetrically positioned.
Another passive system is shown in Canadian Patent No. 565,535 and Great Britain Patent No. 730,594. Both disclose a steering roller made of a flexible material having a plurality of spaced circumferential grooves and rigid annular flange bearings against each end of the roller. If a belt obtains an off-center position, forces within the roll cause a restorative force to be applied to the belt which provides passive corrective action.
U.S. Pat. No. 4,397,538 discloses a belt alignment apparatus which controls a pivoted roller. A spring contacting flanged members resiliently urges a flanged member against one side of a belt being aligned. One of the flanged members then frictionally rotates a disc which tilts the roller to control belt alignment.
U.S. Pat. No. 4,221,480 discloses a belt supporting and alignment system which includes a roller having spaced, flexible discs. Edge guides are located on each end of the roller. If the belt shifts enough to contact one of the guides, the belt bends causing the disks to shift and realign the belt.
The most common active steering method is to introduce corrective skew through a small rotation about the "soft-axis" of one or more guide rolls. This usually involves a yoke arrangement that may be expensive to build and architecturally awkward. While precise control can usually be achieved, this method is less attractive for rigid belts with a large width to length ratio or rigid belts having a high tension. Both of these conditions limit the degree with which the belt path can be twisted or skewed to correct any misalignment. Additionally, it is important in some imaging processes or confined architectures that the surface of the belt remain in a fixed path. This is not possible with the yoke type steering method because the belt geometry must be dynamically twisted or skewed to affect the corrective action. An example of such an active steering method in U.S. Pat. No. 5,078,263 to Thompson et al.
There is a need for a low cost, active steering mechanism which is capable of actively correcting lateral belt misalignment, correcting for "belt walk" (lateral displacement) and "walk rate" (lateral velocity) without reorienting the axis of any roll or spatially altering any portion of the path of the belt being steered.